Research led by Keith A. Olive, professor of physics at the University of Minnesota Twin Cities, is prompting cosmologists to reconsider a long-standing assumption about the early Universe: that dark matter must have been cold at the moment it formed. New theoretical work suggests that dark matter may instead have emerged in an extremely hot, fast-moving state shortly after the Big Bang, while still cooling in time to support the formation of galaxies.’
Henrich, S. E., Mambrini, Y., & Olive, K. A. (2025). Ultrarelativistic Freeze-Out: A Bridge from WIMPs to FIMPs. Physical Review Letters, 135(22), 221002. https://doi.org/10.1103/zk9k-nbpj
Dark matter accounts for most of the matter in the Universe, yet it does not interact with light and can only be detected through its gravitational effects. For decades, models of cosmic structure formation have relied on the idea that dark matter particles were already slow-moving, or “cold,” when they decoupled from the radiation-dominated early Universe. This assumption was driven by concerns that fast-moving particles would smooth out matter distributions and prevent galaxies from forming.
Keith A. Olive, professor of physics at the University of Minnesota Twin Cities stated,
“With our new findings, we may be able to access a period in the history of the Universe very close to the Big Bang”.
Researchers from Minnesota and Université Paris-Saclay examine an alternative timeline. Their analysis focuses on a brief period known as post-inflationary reheating, which followed cosmic inflation and marked the transition to a Universe filled with particles and radiation. During this phase, dark matter could have formed while still traveling at near-relativistic speeds.
Rather than requiring dark matter to slow down before decoupling, the study explores a scenario known as ultrarelativistic freeze-out. In this framework, dark matter particles separate from ordinary matter while still extremely energetic but then lose momentum as the Universe expands. By the time galaxies begin to form, the particles behave effectively as cold dark matter, satisfying observational constraints.
The idea builds on lessons learned from neutrinos, which were once considered promising dark matter candidates. Because neutrinos move rapidly, early studies showed that they would erase small-scale cosmic structure, leading researchers to reject hot dark matter models in favor of cold ones. However, the new work shows that this conclusion depends strongly on when dark matter forms. Particles produced sufficiently early can cool through cosmic expansion long before structure formation begins.
Stephen Henrich, a graduate researcher at Minnesota and lead author of the study, explains that what matters is not the initial temperature of dark matter, but whether it has enough time to cool. In the proposed scenario, dark matter particles formed during reheating experience significant redshifting as space expands, reducing their velocities well before they influence galaxy formation.
The study also helps bridge different theoretical classes of dark matter candidates, linking weakly interacting massive particles (WIMPs) and feebly interacting massive particles (FIMPs) within a continuous framework. This broader view may help explain why direct detection experiments have yet to identify a definitive dark matter signal.
Co-author Yann Mambrini, professor at Université Paris-Saclay, notes that the work connects particle physics more directly with early-Universe cosmology. By tying dark matter properties to reheating, the model offers a way to probe epochs of cosmic history that are otherwise difficult to access observationally.
From an engineering and physics perspective, the findings have implications for how dark matter searches are designed. If dark matter originated hot and cooled later, its present-day interactions could differ from those predicted by conventional cold dark matter models. This may influence strategies for collider experiments, underground detectors, and astrophysical observations.
Rather than overturning cold dark matter as a successful description of today’s Universe, the study refines the conditions under which it can arise. It suggests that the early Universe allowed for a wider range of particle behaviors than previously assumed, without conflicting with observed cosmic structure.
By revisiting assumptions about dark matter’s origins, the research highlights how theoretical advances continue to reshape our understanding of the Big Bang and its aftermath. As experimental techniques improve, these revised models may help guide future efforts to uncover the nature of one of the Universe’s most elusive components.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
